1. Field of the Invention
This invention relates generally to the field of oil well, gas well, water well and subterranean pollution remediation well equipment, and more particularly to downhole screen and more particularly to downhole screens whose design and use are directed to relatively shallow wells, primarily wells which are shallow and/or substantially horizontal or very slightly inclined. A function of such screens is to filter sand and undesirable solids and toxic extractions from fluids and gases in subterranean locations.
2. Brief Description of the Prior Art
A. Sand production is a major problem which has plagued the petroleum industry for many years, as it plagued the water well industry long before men began drilling for oil. The petroleum industry spends millions of dollars annually cleaning sand out of wells, repairing problems related to sand production and loses additional millions of dollars by the resultant restricted production rates.
The production of sand continues to be a worldwide problem. Areas of major problems include not only the United States, the Gulf Coast, California, Canada and the North Sea, but also Europe, China and South America. Unchecked production of sand is very expensive in terms of additional operating expenses, loss revenues, and creation of potentially hazardous situations. Some of the specific problems associated with sand production are as follows:
1. Sand filled and bridges inside the hole, casing or tubing which reduces or shuts off production. PA1 2. Erosion damage to downhole tubulars and equipment including safety valves, chokes and artificial lift equipment. PA1 3. Sand accumulation in surface lines and equipment. PA1 4. Creation of void spaces behind casing leaving the casing unsupported and subject to buckling. PA1 5. Abrasive wear on surface controls, valves and pipes. PA1 6. Increased compressive loading on the casing or liner as subsidence occurs, ultimately causing the casing to either buckle or collapse. PA1 7. Additional expenses associated with sand production include clean out and proper disposal of sand and work over costs to return wells to production. PA1 8. The loss revenue due to restricted or completely shut-in production is often a "hidden" expense which is not always considered. PA1 1. Stop sand movement and production of sand. PA1 2. Maintain maximum well productivity. PA1 3. Payout the treatment costs within a reasonable time.
Obviously, the results of sand production can be extremely expensive if it causes or creates a major setback such as a well blowout or a platform fire.
The production of sand in oil and gas wells is therefore expensive and everything possible is done to successfully control the formation of sand. The goal of all sand control treatment therefore is to stop sand production while maintaining or maximizing fluid production. Thus the success or failure of a sand control treatment must be measured against three interrelated criteria:
Most of the early research work done by the petroleum industry has been directed at a means of controlling sand using the largest practical sizes of gravels and liners with the notion that this would also provide the highest production rates. Research efforts over the years have been aimed at complete stoppage of formation sand while maintaining maximum fluid production and at gravel packing high angle and horizontal wells. The methods for controlling formation of sands can be generally classified as either mechanical, chemical, or combination of mechanical/chemical. The mechanical methods of sand control prevent sand production by stopping the formation of sand with lines, screens or gravel packs.
B. Many types of screen and filtering devices or well liners are known in the prior art that are designed to exclude sand and other solids from fluids and gases produced from oil, gas, water and pollution remediation wells without undue restriction of the production rate of fluids or gases. Well liners are sometimes used as the sole means of filtering sand from the fluids wherein the screen openings are sized to stop the movement of the formation sand and allow continual flow of fluid into the wellbore. Gravel or other particulate matter is also used in conjunction with screens such as "gravel pack" well liners wherein the gravel or particulates are sized to restrain movement of the formation sand and the screen openings are designed to restrain the movement of the gravel or particulates to allow continual flow of fluids therethrough.
Common well liners employ a base pipe having a plurality of openings through the sidewall which is surrounded by longitudinally extending spacer bars, rods, or rings and over which a continuous wire is wrapped in a carefully spaced spiral configuration to provide a predetermined constant gap between the wire wrapping. The gap allows fluids to flow through the wire wrapping and retains the movement of particulate materials such as sand or gravel. Such a well liner is also commonly referred to as a "wire wrapped" screen.
Most "wire wrapped" well liners have the common feature of an "inner annulus" or space between the base pipe and the wire wrapping. The inner annulus is desirable because it allows free transmission of fluids that flow through the wire wrapping gaps into the apertures on the base pipe. The absence of the inner annulus would significantly restrict fluid flow rates radially into the well liner.
The inner annulus in a wire wrapped well liner causes several serious problems during installation of a gravel pack and the stimulation of a well liner or gravel packed well. Problems occur in bypassing fluid through the inner annulus as the gravel is being transported down the "outer annulus", or the gap between the well liner and the wellbore, and bypassing fluid being injected from a well liner through a gravel pack into the formation to stimulate the well or seal a portion of the wellbore.
Bridges of gravel are in part created, or enhanced, as the gravel carrying fluid flows into the inner annulus. As the gravel carrying fluid moves, an increase of gravel concentration in the fluid in the outer annulus results because the gravel is restricted from entering the wire wrapped well liner but fluid enters freely. Thus, when the gravel concentration increases beyond a critical magnitude, a bridge is formed which wedges in the outer annulus and halts further movement of gravel before the outer annulus is fully packed with gravel.
Another problem results from the fluid freely entering the inner annulus from the outer annulus during gravel packing in highly deviated wellbores. The problem is commonly referred to as "duning". In wellbores having angles of 45 degrees to 90 degrees plus from the vertical, and especially those requiring the gravel to be packed along intervals ranging from several feet to more than a thousand feet, the gravel tends to fall to the low side of the wellbore due to gravity as it is being transported by the fluid. As gravel accumulates, the fluid is diverted to the high side of the wellbore and into the well liner, thereby reducing the velocity of carrier fluid in the outer annulus and the capability of the fluid to force gravel toward the bottom end of the wellbore.
The present invention however is directed to different circumstances, namely where there exists horizontal and/or near horizontal wells. In these types of screens, the top of the screen necessarily remains solid, i.e. it is without preparation, holes, apertures, other screen structure. Only the lower half, or less, of the screen is actively designed to perform a screening function. In this way the solid upper half of the structure restricts gravel and other material from falling to the low side of the wellbore due to gravity.
Thus the thrust of the present invention resides in a screen to be used in a cased hole horizontal wellbore perforated only on the bottom of the casing. The blank, or sealed portion of the screen is oriented toward the perforated side of the casing thereby protecting the screen from erosion by direct flow of produced fluids. Gravity will force gravel or sand to the opposite side of the casing and help pack the gravel or sand. This will reduce the tendency of the sand to be produced into the screen openings, as shown hereafter.
No known art deals with shallow and/or substantially horizontal wells for removing pollutants in the same manner as described herein. Despite the fact that a substantial volume of patented technology exists in the field of downhole screens. Attention may be directed to the earlier patent of Sparlin, U.S. Pat. No. 4,771,829 and to the patents identified therein and to the related prior art, none of which appear to provide for a downhole screen particularly utilized for shallow wells and/or near horizontal wells.